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Crop diversification and other interventions can contribute to ensuring sustainable water use in agriculture, writes VK Arora20 May 2019 | 7:19 AM

THE wheat-paddy cycle is the mainstay of Punjab’s agriculture. The two crops accounted for over 80 per cent of nearly 8 million hectares of the gross cultivated area in 2017-18.

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THE wheat-paddy cycle is the mainstay of Punjab’s agriculture. The two crops accounted for over 80 per cent of nearly 8 million hectares of the gross cultivated area in 2017-18. Coupled with high productivity (4 tonnes per hectare of paddy and 5 tonnes per hectare of wheat), the cropping pattern has helped to produce over 28 million tonnes of foodgrains in the state. It has not only improved the economy of the state’s farmers, but also contributed towards the Central pool to ensure the nation’s food security. However, this phenomenal agricultural growth over the years has over-stressed water resources, as indicated by the alarming fall in groundwater levels. The area under paddy has increased nearly 10 times since the early 1970s and crossed 30 lakh hectares in 2016. Paddy has large irrigation needs due to high percolation rates of water in alluvial soils of the state. Advancement of the transplantation date from June-end (in mid-1980s) to mid-May (in mid-1990s) further increased the irrigation needs due to hot and dry weather conditions. This led to a drastic depletion of groundwater resources, with the water table falling in all but a few blocks in south-western Punjab. The average fall in the water table rose from 0.18 metre per year in 1982-87 to 0.74 metre per year in 2004-05. The enactment of the Punjab Preservation of Subsoil Water Act in 2009 (that banned transplanting of rice before June 10) has been a right step to arrest the slide. As a consequence, the average decline in the water table in the past six years (2013-18) was 0.37 metre per year; the badly hit areas of the state (Barnala, Fatehgarh Sahib, Moga, Patiala and Sangrur) experienced a fall of 0.49 metre per year.

Of the 138 blocks that were studied, 109 blocks are over-exploited, two are critical, five semi-critical and only 22 are safe. In terms of groundwater quality, 50-60 per cent of the waters down to a 60-metre depth in the state are fit, 20-30 per cent are marginally fit, and 15-25 per cent are unfit even for irrigation. A continuous decline of groundwater levels caused a large-scale replacement of centrifugal pumps with submersible pumps that have greater energy needs for extraction of water from deeper aquifers. In 2007, the state government provided 7.5 billion units (kilowatt-hours) of energy to extract groundwater; it is likely to be doubled by 2023. Thus, it is imperative to optimise water and energy use for sustaining Punjab’s agriculture.

Regional variations

Water use is determined by its demand and supply. In terms of water supply, the state has three regions: (i) North-east kandi region has mean annual rainfall of 800-1,000 mm with deep, good-quality groundwater, and sloping lands which are prone to water erosion; (ii) Central flood plains record a rainfall of 500-600 mm, with less canal water but good-quality groundwater; this region faces severe problem of declining water table due to intensive paddy culture; and (iii) South-west region, which records rainfall of 300-400 mm, more canal water due to poor-quality groundwater and faces problems of salinity and waterlogging.

A study by PAU experts led by Prof SS Prihar in the early 1990s estimated that Punjab had an annual utilisable water supply of 31.3 billion cubic metres (BCM) from canal networks (14.5 BCM) and groundwater recharge (16.8 BCM). The canal water availability has remained almost static since then; rather, there has been a decline due to reduced river flows. But groundwater recharge and availability registered an increase from 16.8 BCM in the early 1990s to 20.4 BCM in 2011, as per a report of the Water Resources and Environment Directorate of Punjab. This rise is attributed to more than doubling of groundwater extraction (during the 20-year period), as indicated by a substantial increase in the number of tubewells to over 14 lakh. More groundwater extraction also means greater irrigation return flow as a component of recharge.

Seasonal drainage

Water demand includes crop evapotranspiration and non-agricultural civic, industrial and power generation use. Irrigation supplements rainfall to meet crop evapotranspiration and drainage. Seasonal drainage in paddy is twice or even more of evapotranspiration; for other upland crops, this component is 10-20 per cent of evapotranspiration. One kg of rice (equivalent to 1.5-kg paddy) has an evapotranspiration requirement of 1,500 litres, but on the basis of the water input (irrigation and rain to meet evapotranspiration and drainage), this requirement swells to 4,500 litres or even more. The irrigation demand relates better with energy needs, while the evapotranspiration demand is appropriate to explain the groundwater fall.

The State Irrigation Plan (of the state Department of Agriculture and Farmers’ Welfare), prepared by Punjab Agricultural University (PAU) in 2017, has an estimate of utilisable water from canals and groundwater recharge at 35.1 BCM; and the annual water demand (of 2015-16 crop area) and non-agricultural use at 44.1 BCM. It reflects a water deficit (demand exceeding supply) of 9 BCM (implying a negative water balance) that is met through overuse of groundwater. This deficit needs to be minimised by augmenting water supply and reducing demand (withdrawal).

Augmenting water supply

Making more canal water available in the central region by rationalising water allowance of canal systems in the state. For example, water allowance is 9 and 14 cusecs per 1,000 hectares in command areas of the Sirhind feeder and Eastern canal, respectively, that are getting affected by waterlogging; it is only 5 cusecs per 1,000 hectares in the command areas of Bist Doab canal system wherein the groundwater levels are declining continuously. Thus, there is a possibility of diverting canal water from the south-west region (where 70 per cent of the area is canal-irrigated) to the central region (where 14 per cent of the area is canal-irrigated). This would help in greater recharge of groundwater from canal seepage.

Harnessing of surplus river waters during the monsoon that otherwise flow to Pakistan, by constructing new headworks at a suitable site on the Sutlej. This will create a large reservoir to store flood waters during the monsoon and enhance water supply.

Harvesting rainwater in the north-east kandi region (that has undulating topography leading to 40 per cent run-off of rainfall) by adopting in situ water conservation measures and constructing a series of water-harvesting structures (check dams) in the form of earthen embankments. This intervention will also boost recharge in the central region by the lateral sub-surface flow of groundwater.

Using skimming-well technology to extract good-quality water layer above poor-quality water aquifers in the south-west region will supplement the canal water supply.

Promoting rainwater harvesting in urban areas receiving high rainfall by putting in grass-saver tiles along the roadside and parking lots. Growing common grass in holes will allow rainwater to percolate rapidly into the soil. This will help in reducing the sewage water load and avoiding flooded conditions on city roads during the monsoon. Renovation of village ponds having a thick layer of sediments rich in organic matter, and diverting the sediments and water to agricultural lands will be a source of plant nutrients and also increase groundwater recharge.

Waste water from civic and industrial use after treatment can enhance the supply for the irrigation of non-food crops. As per the waste water use policy document (2017), the state has the potential to treat over 2 billion litres per day of waste water through a network of more than 200 treatment plants by state agencies, of which 73 have been completed and commissioned.

These augmentation measures, however, are costly and time-consuming.

Reducing withdrawal

In the central region hit hard by groundwater depletion, field interventions to reduce crop evapotranspiration can be more effective to minimise the water deficit. Measures like diversifying from rice to maize, basmati and kharif pulses having low evapotranspiration needs, and from wheat to rabi oilseeds and gram, can be useful. Synchronising the transplanting time of paddy with low atmospheric evaporative demand will help to reduce evapotranspiration. During April-June in Punjab, there is high atmospheric demand for water, but low rainfall. Thus, crops grown in this period will have very high evapotranspiration and irrigation needs. The wheat-paddy cycle generates huge amounts of residue (more than 45 million tonnes annually), of which paddy residue is generally disposed of through burning. However, mulching with surplus paddy residue helps to reduce soil water evaporation, moderate soil temperature and control the growth of weeds. These three interventions are expected to have a big role in irrigation-saving by reducing evapotranspiration and drainage. They are useful both in terms of energy-saving and curbing the fall in the groundwater level.

A replacement of 10 lakh hectares of the area under paddy from the present 30 lakh hectares to maize (0.5 million hectares), basmati (0.4) and kharif pulses (0.1) can reduce evapotranspiration by 2 BCM. These measures are purposeful when alternative crops have assured and remunerative marketing.

Replacement of 10 lakh hectares of the area under wheat from the present 35 lakh hectares to rabi pulses (gram) and oilseeds (mustard and raya) will reduce evapotranspiration by 1 BCM. This replacement may become necessary in the foreseeable future in view of the improved wheat production in eastern India.

The use of shorter-duration cultivars and a shift in the start of transplanting date from June 10 to June 20 would cause an evapotranspiration saving of 2 BCM for paddy area of 20 lakh hectares.

Residue mulching during hot and dry periods in wide-row crops (for 50 per cent area under 0.5 million hectares of cotton, 0.65 million hectares of maize including area replaced from paddy, and 0.1 million hectares of sugarcane) and residue-retained wheat (1 million hectares) using no-till seeders will result in reducing evapotranspiration by 1.5 BCM.

Drip irrigation in 50 per cent of 0.25 million hectares under fruit and vegetable crops will save evapotranspiration by 0.25 BCM.

Water deficit

These measures can reduce the water deficit by 6.8 BCM from the current 9 BCM. This is equivalent to a decrease in the water table fall by 75 per cent of the existing rate. Furthermore, annual evapotranspiration saving of 6.8 BCM (4 BCM for paddy and 2.8 BCM for other crops) gets translated into irrigation-saving of 13 BCM using drainage-to-evapotranspiration ratio of 1.5 (rice) and 0.10 (other crops). Resource conservation technologies like laser land-levelling, ridge-furrow planting and improved irrigation scheduling (for paddy) cause irrigation-saving due to a reduction in drainage with little effect on crop evapotranspiration.

Laser land-levelling in 1 million hectares under paddy will result in irrigation-saving of 3 BCM (assuming a gain of 20 per cent in application efficiency).

Furrow irrigation in 0.5 million hectares under cotton will result in irrigation-saving of 0.5 BCM.

Thus, a total irrigation-saving of 16.5 BCM implies energy-saving of more than 2.4 billion units (about 20 per cent of energy use in groundwater extraction). Irrigating salt-tolerant crops with poor-quality sodic groundwater in conjunction with good-quality canal water or with amendments (gypsum, farmyard manure and crop residue) in the south-west region can sustain productivity and supplement water supply.

The adoption of these interventions can contribute to sustainable water use in Punjab’s agriculture. The state extension functionaries should play a pro-active role in sensitising the farming community. There is also a dire need for a policy shift from input (energy) subsidy to development subsidy (in terms of conservation of water and improvement in soil health), as advocated by Prof SS Johl, an eminent agricultural economist. This will be a right step to dissuade farmers from the wasteful use of water.

Reducing evapotranspiration

In Punjab’s central region hit hard by groundwater depletion, field interventions to reduce crop evapotranspiration can be effective in minimising water deficit.

Evapotranspiration is the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants.

Measures like diversifying from rice to maize, basmati and kharif pulses — having low evapotranspiration needs — and from wheat to rabi oilseeds and gram are recommended.

Synchronising the transplanting time of paddy with low atmospheric evaporative demand will help to reduce evapotranspiration. During April-June, there is high atmospheric demand for water, but rainfall is low. Thus, crops growing in this period will have very high evapotranspiration and irrigation needs.

Wheat-paddy cycle generates huge amounts of residue (more than 45 million tonnes annually), of which paddy residue is generally disposed of through burning. However, mulching with surplus paddy residue helps to reduce soil water evaporation, moderate soil temperature and control weeds.

The author is a retired professor, Department of Soil Science, PAU, Ludhiana

The average decline in the water table in the past six years (2013-18) was 0.37 metre per year; Punjab’s badly hit areas — Barnala, Fatehgarh Sahib, Moga, Patiala and Sangrur — experienced a drop of 0.49 metre per year.

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